1. Field of the Invention
The present invention relates to communication of a data signal over a power distribution system. More particularly, the present invention relates to a use of an inductive coupler for coupling of a data signal via a conductor in a power transmission cable.
2. Description of the Related Art
In power line communication (PLC), a data coupler couples a data signal between a power line and a communications device, such as, for example, a modem. Radio frequency (rf) modulated data signals can be coupled to and communicated over medium and low voltage power distribution networks.
An example of such a data coupler is an inductive coupler. A power line inductive coupler is basically a transformer whose primary is connected to the power line and whose secondary is connected to the communications device, such as the modem. Examples of inductive couplers and their use are described in U.S. Pat. No. 6,452,482, U.S. patent application Ser. No. 10/429,169 and U.S. patent application Ser. No. 10/688,154, all of which are assigned to the assignee of the present application, and the disclosures of which are incorporated herein by reference.
The inductive couplers achieve a series coupling, which is capable of launching PLC signals with frequencies from below 4 megahertz (MHz) through in excess of 40 MHz along overhead and underground power cables. Unfortunately, in most cases, the power line wires cannot be interrupted. This limits, to a “single turn winding”, the primary winding passing through the inductive coupler. Where the power line impedance is higher than the modem impedance, impedance matching in the data coupler is difficult because while the primary winding is limited to the single turn, the secondary winding cannot be less than a single turn.
Magnetic circuits including inductive couplers exhibit non-linear properties, such as the non-linearity of the circuit's Magnetic Flux Density vs. Applied Magnetizing Force (B-H) curve. This non-linearity, in conjunction with the magneto-motive force rising from zero to a maximum, twice each cycle of the power frequency, causes distortion. The distortion includes amplitude modulation of the transmitted and received signals. The modem or other communication device will begin to suffer data errors at some threshold level of this distortion.
Accordingly, there is a need for an inductive coupler and a corresponding circuit that improves impedance matching between the power line and the communication device or modem. There is a further need for an inductive coupler that reduces distortion of the transmitted and received signals. The apparatus and method of the present invention provides for series coupling of a data signal via a conductor and circuit on a power transmission cable that improves impedance matching and reduces distortion of the signals.